A surface of an aluminum alloy plate is subjected to a surface treatment process including surface roughening treatment and anodizing treatment to obtain a support, a photosensitive material is then applied to the support and dried to obtain a so-called PS plate, the PS plate is then subjected to a plate-making process including image exposure, development and gum coating, and the resulting plate is generally used as an offset printing plate or lithographic printing plate. In such a plate-making process, part of the photosensitive layer which remains undissolved after development forms an image area, whereas the other part of the photosensitive layer which was removed to expose the aluminum surface under the photosensitive layer serves as a water-receptive portion because of its hydrophilic nature, thus forming a non-image area.
A lightweight rolled aluminum alloy plate having excellent surface treatment properties, workability and corrosion resistance is used as a support for offset printing or lithographic printing. Rolled aluminum alloy plates with a thickness of about 0.1 mm to about 0.5 mm made of materials such as JIS1050, JIS1100 and JIS3003 materials have been conventionally used for this purpose, and such rolled aluminum plates are used as printing plates after having undergone surface roughening treatment and its subsequent anodizing treatment.
Specifically, an aluminum lithographic printing plate obtained by carrying out mechanical graining treatment, chemical etching treatment and anodizing treatment in this order as described in Patent Literature 1; an aluminum lithographic printing plate obtained by carrying out electrochemical treatment, post-treatment and anodizing treatment in this order as described in Patent Literature 2; and an aluminum lithographic printing plate obtained by carrying out chemical etching treatment and anodizing treatment in this order as described in Patent Literature 3 are known.
Patent Literature 4 describes that defects of the photosensitive layer during storage can be suppressed by using an aluminum alloy support for a lithographic printing plate in which the number of aluminum carbide particles with a maximum length of 1 μm or more in a rolled plate made of a pure aluminum alloy is regulated to 30,000 pcs/g or less.
Patent Literature 5 describes that a presensitized plate having high adhesion to the photosensitive layer and a long press life can be obtained using a support containing intermetallic compound particles with a diameter of 0.1 μm or more in an amount of 5,000 to 35,000 pcs/mm2.
Patent Literature 6 describes a presensitized plate excellent in laser exposure suitability and printing performance which uses a support obtained by surface-roughening an aluminum plate containing intermetallic compound particles within a depth of 2 μm from the surface at a density of 500 to 35,000 pcs/mm2.
Patent Literature 7 describes obtaining a presensitized plate using a support having a uniformly roughened surface and obtained from an aluminum alloy which includes intermetallic compound particles dispersed in the planar direction at a density of 3,000 to 30,000 pcs/mm2.
However, use of these supports was not enough to prevent the photosensitive layer from having defects during the storage of the resulting presensitized plates.
The plate-making step in conventional PS plates involves an operation for removing the non-image areas by dissolution in a highly alkaline developer after exposure to light and to simplify or eliminate such an additional wet process is a concern which is desired to be resolved for improvement over prior art. Especially in recent years, disposal of wastewater discharged in a wet process is a great concern in the whole of the industry with consideration for the global environment and therefore improvement in this aspect is further strongly requested.
On the other hand, as another recent trend in this field, digitization technology which includes electronic processing, storage and output of image information using a computer has been widely spread and various new image output systems compatible with the digitization technology have been put to practical use. This situation has drawn attention to computer-to-plate (CTP) technology in which a highly convergent radiation such as a laser beam is caused to carry digitized image information and a presensitized plate is scanned and exposed with this beam to directly manufacture a printing plate without using a lith film.
Among others, in order to solve the problem of wastewater treatment while achieving further step rationalization, researches have been made on a presensitized plate that can be directly mounted on a printing press after exposure to light without any development process and be used in printing, and various methods have been proposed therefor.
One of the methods for eliminating a treatment step is a method called “on-press development” in which an exposed presensitized plate is mounted on a plate cylinder of a printing press and fountain solution and ink are supplied as the plate cylinder is rotated to thereby remove non-image areas of the presensitized plate. In other words, this is a system in which the exposed presensitized plate is mounted on a printing press without any further treatment so that development completes in the usual printing process. The presensitized plate suitable for use in such on-press development is required to have an image recording layer which is soluble in fountain solution or an ink solvent and to have a light-room handling property capable of development on a printing press placed in a light room.
For instance, Patent Literature 8 describes a presensitized plate in which an image recording layer containing fine particles of a thermoplastic hydrophobic polymer dispersed in a hydrophilic binder polymer is provided on a hydrophilic support. Patent Literature 8 describes that the plate can be mounted on a plate cylinder of a printing press to remove unexposed areas using fountain solution and/or ink (on-machine development can be made) after an image has been formed by exposing the presensitized plate to a laser beam to coalesce together the thermoplastic hydrophobic polymer particles in the image recording layer by heat. The presensitized plate is photosensitive to the infrared region and therefore also has a light-room handling property.
Patent Literature 9 describes that a presensitized plate having an image recording layer which contains a thermoplastic particulate polymer and at least one of a particulate polymer having a heat-reactive group and a microcapsule containing a compound having a heat-reactive group has good on-press developability, a high sensitivity and a long press life.
However, in cases where the presensitized plates described in Patent Literatures 8 and 9 are stored for a long period of time, ink is prone to adhere to part of the non-image area surface, causing dot- or ring-shaped stains on printed paper. This defect is also hereinafter referred to as “corrosive micro-stains.”
As for the CTP technology, high-power lasers including a semiconductor laser and a solid-state laser such as a YAG laser are recently available at low cost and therefore a plate-making method using high power density exposure with a high-power laser is highly promising.
Of those presensitized plates, one of the promising means suitable to a simple development process is a presensitized plate having an image recording layer which is a hydrophilic layer obtained by dispersing hydrophobic thermoplastic polymer particles in a hydrophilic binder polymer. This is the means which makes use of the conversion of the surface of the hydrophilic layer to a lipophilic image area due to fusion of the hydrophobic thermoplastic polymer particles upon exposure of the image recording layer.
Each of Patent Literatures 10 and 11 describes a method of making a lithographic printing plate which involves developing an image recording layer having hydrophobic thermoplastic polymer particles by a simple development process.
However, it has been difficult to sufficiently remove the image recording layer in non-image areas containing the hydrophobic thermoplastic polymer particles by a simple development means and the ingredients of the image recording layer remain in the non-image areas to cause stains in printing. In cases where a plate is stored for a long period of time, ink is prone to adhere to part of the non-image area surface, causing dot- or ring-shaped stains (hereinafter also referred to as “corrosive micro-stains”) on printed paper.
Lithographic printing plate materials each containing a binder resin soluble in an aqueous alkali solution and an infrared absorbing dye generating heat by light absorption as essential ingredients are known as positive type recording materials compatible with an infrared laser having a photosensitive wavelength of 760 nm or more (see, for example, Patent Literatures 12 to 15).
Upon exposure of such a positive type recording material compatible with an infrared laser to infrared laser radiation, in unexposed areas (image areas), the solubility of the binder resin is substantially reduced by the interaction between the infrared absorbing dye or the like and the binder resin, and in the exposed areas (non-image areas), the infrared absorbing dye or the like absorbs light to generate heat, thus reducing the interaction between the infrared absorbing dye or the like and the binder resin. As a result, the exposed areas are only dissolved in the alkali developer and removed during development, whereby a lithographic printing plate is formed.
However, in such a positive type recording material compatible with an infrared laser, the exposed areas (non-image areas) are not dissolved in the alkali developer and part of the film remains in a spot shape (this defect is hereinafter referred to as “residual film spots”), which may cause printing stains.
In the development using an automatic developing machine, an exposed presensitized plate is repeatedly immersed in a development bath containing an alkali developer but aluminum scum (hereinafter referred to as “development scum”) formed in the development bath during the passage of a number of presensitized plates through the automatic developer may be adhered to non-image areas of a lithographic printing plate to cause stains.